1. Field of the Invention
This invention lies in the field of sulfur removal and recovery, and particularly in the treatment of sour gases and other gases in the chemical process industry that contain hydrogen sulfide.
2. Description of the Prior Art
Much of the natural gas produced in the United States has a hydrogen sulfide content exceeding 4 parts per million and is therefore classified as "sour." Since hydrogen sulfide is an environmental hazard, sour natural gas is considered unacceptable for transport or use. Hydrogen sulfide levels are also a problem in the fabrication of fuels derived from petroleum, coal and lignite, whose sulfur content is converted to hydrogen sulfide during the conversion of these materials to gasoline, jet fuels, refinery gas, coal gas, blue-water gas and the like. In addition to the environmental hazard, hydrogen sulfide represents a loss of the sulfur value of the raw material, which if recovered as elemental sulfur would be of significant use to the chemical industry.
The traditional method for converting hydrogen sulfide in natural gases and in gaseous plant effluents is the Claus process, in which part of the hydrogen sulfide is burned in air to form sulfur dioxide and water: EQU 2 H.sub.2 S+3 O.sub.2 .fwdarw.2 SO.sub.2 +2 H.sub.2 O (A)
and the sulfur dioxide thus produced is reacted with further hydrogen sulfide over an alumina catalyst to form sulfur and additional water: ##EQU1##
The furnace (Reaction A) in the Claus process is operated with a fuel-rich mixture, converting only one-third of the H.sub.2 S to SO.sub.2. The fuel-rich atmosphere results in the partial conversion of hydrocarbons that are present in the H.sub.2 S feed to such compounds as COS and CS.sub.2, which lessen the yield of elemental sulfur and are themselves hazardous. The fuel-rich atmosphere also promotes the breakdown of aromatics to soot. For high sulfur recovery, precise control of the overall stoichiometry is needed, and this is made especially difficult when considerable amounts of CO.sub.2 and other inerts are present.
Part of Reaction B occurs in the furnace and the rest is conducted in a heterogeneous system in which the reaction mixture is gas-phase and contacts a solid activated alumina catalyst of a sort well known to those skilled in the art of the Claus process. With continued use, the alumina catalyst fouls and becomes otherwise deactivated over time. This requires plant shutdown, loss of process time, and the cost of regeneration or replacement of the catalyst, together with the associated labor costs.
A further disadvantage of Reaction B is that it is equilibrium-limited at temperatures above the dewpoint of sulfur, and despite being performed in two to four stages, the reaction leaves 2% to 5% of the H.sub.2 S and SO.sub.2 unreacted. Each stage requires a separate condenser to remove the elemental sulfur, and these condensers require a large heat-exchange area and reheating of the gas leaving each but the last condenser. Furthermore, the steam generated by each condenser is low in pressure, limiting its usefulness. Additional costs are entailed in treating the tail gas in which the sulfur content must be reduced by ten to twenty times.